1. Field of the Invention
The invention relates to multimode infrared detectors and, more particularly, to a novel optical coupling apparatus for such detectors.
2. The Prior Art
Prior infrared measurement systems for "multimode" sources, i.e., sources whose image sizes are not limited by diffraction, generally suffer disadvantages with respect to efficiency and speed of operation. A multimode source can be defined a source for which the throughput of the infrared beam is much larger than a square wavelength, a relationship which can be represented by the equation A .OMEGA.&gt;&gt;.lambda..sup.2, wherein A is the cross-sectional area of the beam, .OMEGA. is the solid angle subtended by the beam and .lambda. is the wavelength of interest. Another way of describing a multimode application is as one in which diffraction-limited spatial resolution is not required and one which concerns only a measurement of average infrared surface temperature, i.e., infrared power per unit solid angle. This excludes many imaging applications such as those involving large telescopes and distant sources (e.g., satellite-based infrared surveillance and astronomical imaging) but includes a wide variety of smaller scale terrestrial applications in, e.g., infrared laboratory instrumentation and industrial process control.
As will appear, the present invention is, broadly speaking, a combination of two units or components which are known per se in the prior art. The first of these is an ideal flux concentrator and such a flux concentrator is described, for example, in Harper et al, "Heat trap: an optimized far infrared field optics systems," Applied Optics, 15, pp. 53-60 (1976). An idea flux concentrator comprises a reflecting metal cone of specific tapered geometry, having a wide end and small end, and backed at the small end by a hollow metal-walled cavity (referred to as an "integrating cavity"). Infrared radiation enters the concentrator through the wide end of the metal cone, and exits through an aperture at the small end of the cone into the integrating cavity. The shape of the cone is that of an off-axis parabola, rotated about the optical axis. In the limit of short wavelength, a concentrator device of this kind achieves the highest degree of infrared flux concentration allowed by the laws of optics. Such cones, known as Winston cones, have been used for many years in conjunction with bolometers, and this is what is disclosed in the Harper et al article referred to above. Such bolometers have high thermal mass and, are therefore, low speed devices, because these bolometers must have absorbing areas much larger than the entrance area of the cavity.
Exemplary patents of interest in the field include U.S. Pat. Nos. 2,984,747 (Walker), 4,479,056 (Zeirhut), 4,754,139 (Ennulat et al), 5,239,179 (Baker), and 5,254,858 (Wolfman et al). The Walker patent discloses an infrared sensor located adjacent to the focus of a cone-shaped radiation collector, with the sensor being positioned within an integrating hemisphere affixed to the bottom of the collector. The Zeirhut patent discloses an infrared radiation responsive motion detector wherein a cone is used to focus radiation on an infrared detector. The Wolfman et al patent discloses the use of a cone non-imaging concentrator for spectroscopy. The Ennulat et al patent discloses an array of flux concentrators for focusing infrared radiation onto a detector array to improve efficiency without cooling the array. The Baker patent discloses an array of non-lens concentrators associated with an infrared detector array.